Synchronous rectification step-down DC-to-DC converters are used as high-efficiency power sources. A switching circuit composed of a high-side switch and a low-side switch, for example, is used in this kind of DC-to-DC converter. However, in the DC-to-DC converter, the proportion of the conduction loss on the low-side switch side is increasing as a factor of the efficiency reduction, due to increases in the ON period and current of the low-side switch caused by a decrease in voltage.